Piezoelectric device

ABSTRACT

A piezoelectric device having an electromechanical coefficient, which can be applied to practical uses, is obtained by an energy-trapping effect using a piezoelectric ceramic having a layered perovskite structure provided with high-temperature resistance and little high-frequency loss. The piezoelectric device has a substrate formed of a piezoelectric ceramic having a layered perovskite structure. The c-axis in the substrate is preferentially oriented in the width direction thereof, and the substrate is polarized in the longitudinal direction. Two electrodes are formed on both main surfaces of the substrate so as to oppose each other in the vicinity of the central part thereof. As a material used for the substrate, a piezoelectric ceramic composition primarily composed of a ceramic composition represented by the general formula CaBi 4 Ti 4 O 15  is preferably used.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to piezoelectric devices, and moreparticularly, relates to a piezoelectric device used for, for example,an oscillator used in a filter for communication and in a clockgenerator.

[0003] 2. Description of the Related Art

[0004] Conventionally, as piezoelectric resonators for oscillators usedin filters for communication and in clock generators, piezoelectricdevices formed of piezoelectric ceramics primarily composed of leadtitanate zirconate (Pb(Ti_(x)Zr_(1−x))O₃) or lead titanate (PbTiO₃) arewidely used to employ the shear vibration of the piezoelectric ceramics.In general, the piezoelectric devices have substrates formed ofrectangular piezoelectric ceramics and electrodes provided on both mainsurfaces of the substrate. The electrodes are not formed on the entiresurfaces of the substrate, but are formed on parts of the surfaces sothat parts of the electrodes oppose each other.

[0005] In the piezoelectric device thus described, by appropriatelyselecting the type of piezoelectric ceramic and the shapes ofelectrodes, a phenomenon can be realized in which the energy of thepiezoelectric vibration of the piezoelectric ceramic is localized at thearea between the electrodes which oppose each other, that is, anenergy-trapping can be realized. Consequently, a single piezoelectricvibration mode can be obtained, and an effective piezoelectric device asa piezoelectric resonator for an oscillator used in a filter forcommunication and in a clock generator can be obtained. However, thepiezoelectric ceramics have problems in that high-temperature resistanceis poor and loss in high-frequency regions is large.

[0006] Piezoelectric ceramics having a layered perovskite structure,such as, CaBi₄Ti₄O₁₅ and PbBi₄Ti₄O₁₅, have features of high-temperatureresistance, small high-frequency loss, and the like, compared to thepiezoelectric ceramics primarily composed of lead titanate zirconate orlead titanate. Hence, the piezoelectric ceramics described above areexpected to be suitable materials used for piezoelectric resonatorswhich can be used under high temperature conditions or in a highfrequency region. However, since the piezoelectric ceramics have stronganisotropic characteristics in the crystal, a high electromechanicalcoefficient cannot be obtained by a general manufacturing method forpiezoelectric ceramics. Accordingly, methods have been proposed in whichthe c-axis in piezoelectric ceramics having a layered perovskitestructure is preferentially oriented in one direction so as to obtain alarge electromechanical coefficient. For example, T. Takenaka, et. al.,reported that 1.6 times the electromechanical coefficient ofpiezoelectric ceramics produced by a conventional manufacturing methodwas obtained in vertical fundamental vibration of a cylindricaloscillator using an orientational ceramic of PbBi₄Ti₄O₁₅ formed by a hotforging method (J. Appl. Phys., vol. 55. No. 4. 15 (1984)).

[0007] In general, in order to obtain a piezoelectric resonator for anoscillator used in a filter for communication and in a clock generator,a single piezoelectric vibration mode having a slight spurious vibrationis necessary. In a piezoelectric device using vertical vibration andshear vibration, a single mode is generally obtained by trapping energyusing opposing electrodes. However, it has been known that when thePoisson ratio of a piezoelectric ceramic is one-third or less, verticalfundamental vibration cannot trap energy. Poisson ratios of almost allpiezoelectric ceramics having a layered perovskite structure, such asCaBi₄Ti₄O₁₅ and PbBi₄T4O_(15,) are one-third or less, so that it isdifficult to trap energy.

[0008] Concerning higher harmonic vertical vibration, since restrictionof a Poisson ratio is not so strict compared to the fundamental wave, itis likely to trap energy; however, in general, the electromechanicalcoefficient is greatly decreased compared to that of the fundamentalwave. Consequently, even though a single vibration mode is obtained,application as a piezoelectric resonator is limited. In contrast, forshear vibration, the electromechanical coefficient is at a levelequivalent to that of vertical vibration, and is not restricted by aPoisson ratio.

[0009] However, no experiments to trap energy of shear vibration havebeen performed using a piezoelectric ceramic having a layered perovskitestructure in which the c-axis is preferentially oriented in onedirection. Even though piezoelectric ceramics having a layeredperovskite structure, such as CaBi₄Ti₄O₁₅ and PbBi₄Ti₄O₁₅, havehigh-temperature resistance and little high-frequency loss, which werenot provided in conventional piezoelectric materials, no piezoelectricresonator for an oscillator used in a filter for communication and in aclock generator, which can be practically used, has been manufactured asyet.

SUMMARY OF THE INVENTION

[0010] Accordingly, a major object of the present invention is toprovide a piezoelectric device having an electromechanical coefficientof not less than 20%, which can be practically used, by anenergy-trapping effect using a piezoelectric ceramic having a layeredperovskite structure provided with high-temperature resistance andlittle high-frequency loss.

[0011] A piezoelectric device of the present invention comprises asubstrate composed of a piezoelectric ceramic having a layeredperovskite structure, and a plurality of electrodes provided at thesubstrate, in which one crystal axis in the substrate is preferentiallyoriented, and the substrate is polarized in an approximately orthogonaldirection to the direction in which the crystal axis is preferentiallyoriented, and the plurality of electrodes are formed on surfaces of thesubstrate which are approximately parallel to the direction in which thecrystal axis is preferentially oriented and are approximately parallelto the direction in which the substrate is polarized.

[0012] In the piezoelectric device described above, the substrate ispreferably formed of a piezoelectric ceramic using a piezoelectricceramic composition primarily composed of a ceramic compositionrepresented by the formula CaBi₄Ti₄O₁₅.

[0013] When the electrodes are formed on the substrate composed of apiezoelectric ceramic having a layered perovskite structure, and whenthe direction in which one crystal axis in the substrate ispreferentially oriented approximately orthogonal to the direction inwhich the substrate is polarized, and the electrodes are formed onsurfaces of the substrate which are approximately parallel to thedirection in which the crystal axis in the substrate is preferentiallyoriented and are approximately parallel to the direction in which thesubstrate is polarized, a device having superior high-temperatureresistance and little high-frequency loss can be obtained. In addition,the device thus obtained has an electromechanical coefficient which canbe practically used.

[0014] In the piezoelectric device described above, when thepiezoelectric ceramic composition is used, which is primarily composedof a ceramic composition represented by, specifically, the formulaCaBi₄Ti₄O₁₅, superior temperature stability of resonant frequency can beachieved.

[0015] The objects described above, other objects, features, andadvantages of the present invention will be apparent from the followingdetailed description of preferred embodiments thereof with reference tothe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a perspective view showing an example of a piezoelectricdevice according to the present invention;

[0017]FIG. 2A is a perspective view showing a baked product in which thec-axis is preferentially oriented;

[0018]FIGS. 2B to 2D are perspective views showing substrates cut fromthe baked product shown in FIG. 2A; and

[0019]FIG. 3 is a perspective view showing a substrate formed in theexample provided with electrodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020]FIG. 1 is a perspective view showing an example of a piezoelectricdevice according to the present invention. A piezoelectric device 10comprises a substrate 12, for example, in the form of a rectangularparallelepiped. As a material for the substrate 12, CaBi₄Ti₄O₁₅ or thelike is used. The substrate 12 has a layered perovskite structure, inwhich the c-axis is preferentially oriented in the width direction ofthe substrate 12 as indicated by a solid arrow. In addition, as shown bya dotted arrow, the substrate 12 is polarized in the longitudinaldirection thereof. A first electrode 14 and a second electrode 16 arerespectively formed on each of the main surfaces in the thicknessdirection of the substrate 12. The first electrode 14 is formed from onelongitudinal end to a central part of the substrate 12. The secondelectrode 16 is formed from the other longitudinal end to a central partof the substrate 12. The first electrode 14 and the second electrode 16oppose each other in the vicinity of the central part of the substrate12. Accordingly, the first electrode 14 and the second electrode 16 areformed on the main surfaces which are parallel to the direction in whichthe c-axis is preferentially oriented (hereinafter referred to as thepreferential c-axis oriented direction), and which are parallel to thedirection in which the substrate is polarized (hereinafter referred toas the polarized direction).

[0021] In the piezoelectric device 10, since the preferential c-axisoriented direction and the polarized direction cross orthogonally, andthe first electrode 14 and the second electrode 16 are formed on thesurfaces parallel to both directions described above, an energy-trappingof shear vibration can be realized, and a single piezoelectric vibrationmode without spurious vibration can be obtained. The electromechanicalcoefficient of the piezoelectric device 10 of the present invention isgreater and a superior rate of change in resonant frequency withtemperature can be obtained, compared to the case in which the c-axis inthe piezoelectric ceramic forming the substrate 12 is not preferentiallyoriented, or in the case in which the relationship between thepreferential c-axis oriented direction and the polarized direction orthe relationship between these directions mentioned above and theelectrodes are different from those described above. Furthermore, sincethe substrate used for the piezoelectric device 10 is composed of apiezoelectric ceramic having a layered perovskite structure, such asCaBi₄Ti₄O₁₅, superior high-temperature resistance, small high-frequencyloss, and the like can be obtained.

[0022] The preferential c-axis oriented direction and the polarizeddirection may cross approximately orthogonal to each other, and whenthey cross at angles not deviating more than 10° with respect to theorthogonal point therebetween, that is, in the range from 80° to 100°,the advantages of the present invention can be obtained. In addition,the first electrode 14 and the second electrode 16 may be formed on thesurfaces approximately parallel to the preferential c-axis orienteddirection and to the polarized direction, and when the electrodesincline at angles of not more than 10° from the surfaces parallel tothese directions, the advantages of the present invention can beobtained.

[0023] Example

[0024] After CaO, Bi₂O₃ and TiO₂ were prepared as starting materials andwere measured so as to form the composition CaBi₄Ti₄O₁₅, the startingmaterials were wet-blended for 4 hours using a ball mill, and a mixturewas obtained. After drying, the mixture was pre-baked at 900° C. and wascoarsely pulverized, and after adding an appropriate amount of anorganic binder to the mixture, wet-pulverization was performed for 4hours using a ball mill. The product thus prepared was sieved with a #40sieve to control particle size. Next, the product was molded at apressure of 1,000 kg/cm² to form a cylinder having a diameter of 20 mmand a thickness of 10 mm, and the cylinder thus formed was heat treatedat 600° C. so as to remove the organic binder and obtain a pretreatedproduct.

[0025] While the pretreated product was pressed in the thicknessdirection thereof at a total pressure of 1 ton by mono-axial pressing,the pretreated product was baked at 1,200° C. for 2 hours to obtain abaked product. When the baked product was evaluated by x-ray analysis,it was confirmed that the baked product in which the c-axis thereofpreferentially oriented along the mono-axial pressing direction wasobtained, as shown by an arrow in FIG. 2A. The baked product was cutinto rectangular substrates, which were 10 mm long, 2.5 mm wide and 0.25mm thick, by three different methods described below. As shown in FIG.2B, a substrate in which two main surfaces thereof were parallel to thec-axis oriented direction (thickness direction of the baked product) anda longitudinal direction was orthogonal thereto was designated assample 1. As shown in FIG. 2C, a substrate in which two main surfacesthereof were parallel to the c-axis oriented direction and alongitudinal direction was parallel thereto was designated as sample 2.Furthermore, as shown in FIG. 2D, a substrate in which two main surfacesthereof were orthogonal to the c-axis oriented direction. In FIGS. 2B to2D, solid arrows indicate the c-axis oriented direction was designatedas sample 3.

[0026] In addition, the pretreated product was baked at 1,200° C. for 2hours in the air. When a resulting baked product was evaluated by x-rayanalysis, no orientation could be observed. The baked product was alsocut into rectangular substrates which were 10 mm long, 2.5 mm wide and0.25 thick, similar to the samples 1 to 3 cut from the baked producthaving the preferential c-axis orientation. A substrate in which twomain surfaces thereof were parallel to the thickness direction of thebaked product and the longitudinal direction was orthogonal thereto wasdesignated as sample 4. A substrate in which two main surfaces thereofwere parallel to the thickness direction of the baked product and alongitudinal direction was parallel thereto was designated as sample 5.A substrate in which two main surfaces thereof were orthogonal to thethickness direction of the baked product was designated as sample 6.

[0027] Silver electrodes were formed on the entire opposing edge facesin the longitudinal direction of the samples 1 to 6 by coating a silverpaste thereon followed by baking, and polarization was performed inwhich direct current of 5 kV/mm was applied to the samples for 1 hour inan insulating oil at 200° C. Accordingly, the substrates were polarizedin the longitudinal directions thereof, that is, the samples 1 to 3 werepolarized in the directions indicated by dotted arrows in FIGS. 2B to2D. After removing the silver electrodes from the samples, as shown inFIG. 3, electrodes were formed on both main surfaces of the substrates.A 7.5 mm-long electrode was formed on one main surface of the substratefrom one edge thereof in the longitudinal direction to a central portionthereof. In addition, a 7.5 mm-long electrode was formed on the othermain surface of the sample from one edge thereof in the longitudinaldirection to a central portion thereof. Accordingly, in an area 5 mmlong in the longitudinal direction at a central part of the sample, twoelectrodes opposed each other.

[0028] Piezoelectric devices were formed by providing electrodes asshown in FIG. 3 on individual substrates of samples 1 to 6, and thepiezoelectric devices thus obtained from the samples 1, 2, 3, 4, 5, and6 are called samples a, b, c, d, e, and f, respectively.Electromechanical coefficients and rates of change in resonant frequencywith temperature (fr-TC) from −20° C. to 80° C. were measured for theresulting samples a to f, and the results are shown in Table 1. The rateof change in resonant frequency with temperature is represented by thefollowing equation,

(fr−TC)={(resonant frequency at 80° C.)−(resonant frequency at−20°C)}/{(resonant frequency at 20° C.)/100 }.

[0029] TABLE 1 Rate of Change in Electromechanical Resonant FrequencyCoefficient with Temperature K (%) fr-TC (ppm) Sample a 28.0 −30 Sampleb Not measurable Not measurable Sample c 5.1 −139 Sample d 12.1 −69Sample e 12.4 −73 Sample f 11.9 −72

[0030] As can be seen from Table 1, for the samples a, and c to f, asingle piezoelectric vibration mode without spurious vibration wasobtained. However, for the sample b, the piezoelectric vibration was soweak that measurement could not be performed. Concerning the samples cto f, electromechanical coefficients K were approximately 5 to 10% andwere not sufficient for practical use. In contrast, the sample a had anelectromechanical coefficient K of 20% or more, and that result wassufficient for practical use. In addition, concerning the sample a, theabsolute value of the rate of change in resonant frequency withtemperature was noticeably less compared to those of the samples c to f.As piezoelectric resonators for oscillators used in filters forcommunication and in clock generators, a small absolute value of rate ofchange in resonant frequency with temperature is preferable. From thispoint of view, the sample a is superior to the samples c to f.

[0031] As has thus been described, in a ceramic having a layeredperovskite structure, when the c-axis is preferentially oriented,polarization is performed in the direction orthogonal to thepreferential c-axis oriented direction, and electrodes are formed onsurfaces parallel to the preferential c-axis oriented direction and tothe polarized direction, an energy-trapping of shear vibration can berealized, and a single piezoelectric vibration mode without spuriousvibration can be obtained. In addition, a greater electromechanicalcoefficient and a superior temperature characteristic of resonantfrequency can be obtained, compared to the case in which the c-axis isnot preferentially oriented, and the conditions described above are notsatisfied. Consequently, the piezoelectric device of the presentinvention has sufficient properties to be used as a piezoelectricresonator for an oscillator used in filters for communication and inclock generators. In addition, shape and size of the electrodes are notlimited to those shown in FIG. 3. When optional shape and size of theelectrodes capable of realizing an energy-trapping of shear vibrationare employed, the advantages described above can be observed.

[0032] Furthermore, electromechanical coefficients and rates of changein resonant frequency with temperature (fr-TC) were measured for sampleshaving angles of 90°, 80° and 70° formed by the preferential c-axisoriented direction and the polarized direction. In addition,electromechanical coefficients and rates of change in resonant frequencywith temperature of the samples described above were also measured inthe case in which angles formed by the preferential c-axis orienteddirection and the electrodes were 0° (parallel), 10° and 20°, anglesformed by the polarized direction and the electrodes were 0° (parallel),10° and 20°, and a combination thereof. The results are shown in Table2. TABLE 2 Relationship Between Preferential Rate of Change Orientation,Polarization, and Electrodes in Resonant Preferential PreferentialElectro- Frequency Orientation Orientation Polarization mechanical Withvs. vs. vs. Coefficient Temperature Polarization Electrodes ElectrodesK(%) fr-TC (ppm) Orthogonal Parallel (0°) Parallel (0°) 28.0 30 (90°)10° 24.2 41 20° 18.2 51 10° Parallel (0°) 23.2 45 10° 22.8 53 20° 14.963 20° Parallel (0°) 16.6 51 10° 13.8 61 20° 13.1 72 80° Parallel (0°)Parallel (0°) 24.8 40 10° 22.6 52 20° 15.8 65 10° Parallel (0°) 22.8 4810° 20.2 59 20° 13.4 72 20° Parallel (0°) 15.8 60 10° 12.7 72 20° 10.378 70° Parallel (0°) Parallel (0°) 19.4 53 10° 14.8 66 20° 13.1 71 10°Parallel (0°) 15.2 61 10° 13.1 72 20° 12.8 75 20° Parallel (0°) 13.4 6710° 12.8 72 20° 12.2 79

[0033] As can be seen from Table 2, when a sample had angles of 90° and80° formed by the preferential c-axis oriented direction and thepolarized direction, angles of 0° and 10° formed by the preferentialc-axis oriented direction and the electrodes, and angles of 0° and 10°formed by the polarized direction and the electrodes, theelectromechanical coefficient was 20% or more and the rate of change inresonant frequency with temperature was also small. In contrast,electromechanical coefficients of other samples were less than 20%. Asdescribed above, when the angles formed by the preferential c-axisoriented direction and the polarized direction was not more than 10°with respect to the orthogonal point therebetween, and when theelectrodes incline at angles of not more than 10° from the preferentialc-axis oriented direction and at angles not more than 10° from thepolarized direction, a great electromechanical coefficient and a smallrate of change in resonant frequency with temperature were obtained.

[0034] As a material for the substrate, piezoelectric ceramics primarilycomposed of compounds having layered perovskite structures provided withdistinguishing anisotropy in the c-axis direction are effectively used.They are, in addition to CaBi₄Ti₄O₁₅, for example, Bi₃TiNbO₉, Bi₄Ti₃O₁₂,PbBi₃Ti₂NbO₁₂, BaBi₃Ti₂NbO₁₂, SrBi₃Ti₂NbO₁₂, CaBi₃Ti₂NbO₁₂, PbBi₄Ti₄O₁₅,SrBi₄Ti₄O₁₅, BaBi₄Ti₄O₁₅, NaO₀ ₅Bi₄ ₅Ti₅O₁₅, K_(0.5)Bi₄ ₅Ti₅O₁₅,Sr₂Bi₄Ti₅O₁₈, Ba₂Bi₄Ti₅O₁₈, Pb₂Bi₄Ti₅O₁₈, Ca₂Bi₄Ti₅O₁₈, Bi₆Ti₃WO₁₈,Bi₇Ti₄NbO₂₁, and Bi₁₀Ti₃W₃O₃₀.

[0035] However, since CaBi₄Ti₄O₁₅ has a specifically high Curietemperature (approximately 790° C.) and superior temperature stabilityamong the compounds having layered perovskite structures, it isparticularly effective to produce a piezoelectric device usingCaBi₄Ti₄O₁₅.

[0036] According to the present invention, using an energy-trappingeffect of shear vibration, a piezoelectric device as an effectivepiezoelectric resonator for an oscillator used in a filter forcommunication and in a clock generator can be obtained. In addition, thepiezoelectric device has an electromechanical coefficient which can beapplied in practical uses and has features such as high-temperatureresistance and small high-frequency loss, which are provided topiezoelectric ceramics having layered perovskite structure. Furthermore,by using CaBi₄Ti₄O₁₅, as a material for piezoelectric ceramics, thepiezoelectric device having a small rate of change in resonant frequencywith temperature can be obtained.

What is claimed is:
 1. A piezoelectric device comprising: a substratecomprising a piezoelectric ceramic having a layered perovskitestructure; and a plurality of electrodes on the substrate; wherein onecrystal axis in the substrate is preferentially oriented, and thesubstrate is polarized in an approximately orthogonal direction to thedirection in which the crystal axis is preferentially oriented, and theplurality of electrodes are formed on surfaces of the substrate whichare approximately parallel to the direction in which the crystal axis ispreferentially oriented and are approximately parallel to the directionin which the substrate is polarized.
 2. A piezoelectric device accordingto claim 1, wherein the substrate comprises a piezoelectric ceramic of aceramic composition represented by the formula CaBi₄Ti₄O₁₅.
 3. Apiezoelectric device according to claim 2, wherein the electrodes do notincline at an angle of more than 10° from the surfaces approximatelyparallel to the direction in which the crystal axis is preferentiallyoriented and are approximately parallel to the direction in which thesubstrate is polarized.
 4. A piezoelectric device according to claim 3,wherein the axis preferentially oriented is the c-axis and thepreferential c-axis oriented direction and the polarized direction crossat an angle not deviating more than 10° with respect to the orthogonalpoint therebetween.
 5. A piezoelectric device according to claim 4,wherein the substrate is approximately rectangular and wherein thec-axis in the substrate is preferentially oriented in the widthdirection thereof and the substrate is polarized in the longitudinaldirection.
 6. A piezoelectric device according to claim 2, wherein theaxis preferentially oriented is the c-axis and the preferential c-axisoriented direction and the polarized direction cross at an angle notdeviating more than 10° with respect to the orthogonal pointtherebetween.
 7. A piezoelectric device according to claim 6, whereinthe substrate is approximately rectangular and wherein the c-axis in thesubstrate is preferentially oriented in the width direction thereof andthe substrate is polarized in the longitudinal direction.
 8. Apiezoelectric device according to claim 1, wherein the substrate isapproximately rectangular and wherein the c-axis in the substrate ispreferentially oriented in the width direction thereof and the substrateis polarized in the longitudinal direction.